Many methods determining the position of an object using a transmitter/receiver device in conjunction with at least one active/reflective device situated at a pre-determined position as a point of reference. Frequently, a system is provided which is capable of monitoring its own position relative to the active/reflective device, and this system is attached to the object.
The best known of such systems is the GPS system, in which the position of a GPS receiver located on the surface of the earth is determined using 27 Earth-orbiting satellites (of which 24 are in operation at any time). The distances between the receiver and at least three of the satellites are determined by measuring the travelling time of radio waves between them. Subsequently, a 3D trilateration method can be applied to determine the receiver's location on the planet (see Marshall Brain & Tom Harris, “How GPS receivers work”, http://electronics.howstuffworks.com/gps.htm). The calculations are dependent on an almanac in the receivers detailing the position of the satellites at any given time.
U.S. Pat. Nos. 5,977,958 and 6,054,950 describe methods for measuring time-of-arrival with ultrashort RF pulses (UWB) transmitted from the transmitter to an array of receivers. Despite the differences in technical details, both patents require devices at pre-known positions. U.S. Pat. No. 5,977,958 uses 4 receivers for its 2D scenario and U.S. Pat. No. 6,054,950 needs at least 4 receivers or beacons for 3D applications (these receivers or beacons are here termed “explicit references”). There are other similar technologies which are based on angles of arrival of received signals instead of their times of arrival. In such technologies, multiple references are needed. The problem with this type of system is the requirement for references at pre-known positions. This increases the number of devices needed and introduces difficulty in setting up the systems in certain situations or surroundings. Furthermore, such systems require direct line of sight (LOS) between the transmitter and receivers, the performance decreases sharply in the areas where LOS is not available.
Self-positioning methods employed in robotics often use a ring of ultrasonic transducers or laser range finders to get a measurement of the respective distance from an object to a surrounding environment in each of a number of directions. A sonar/laser range image can be formed from the distances between the detector and nearby objects. This image can be compared to a known floor plan or trained database in order to find the location of the robot. However, these methods involve multiple directional sensors (e.g. a ring of ultrasonic sensors around the robot) or scanning mechanism (e.g. a scanning laser range finder) and are highly complex.
Cameras are also used for positioning purposes. Optical images of surroundings captured at a certain location with a certain orientation (or information abstracted from these images subsequently) can be compared to a database, to determine the current position and bearing of the robot. However, such systems can only work in special environments where specific visible features (e.g. straight lines) are available. Another disadvantage is that daunting computing resources are needed for image processing in these systems.
Another U.S. Pat. No. 6,112,095 proposes a method of location determination in which a transmitter transmits a radio signal and a receiver device uses an array of antennas to receive the signal both directly and along paths which include reflections (multipath signals). Different locations of the transmitter cause the set of reflected paths to be different, so that the received signals constitute a signature of the position. The determination of the location of the transmitter is based on a set of pre-calibrated signal covariance matrices corresponding to possible transmitter locations. However, the uniqueness of the signature is, in principle, not guaranteed and sometimes leads to big errors when the multipath features of one location are similar to those of other locations. Furthermore, the teaching in this citation is meant for outdoor application, and a base-station is required for its implementation.